The present invention relates to a process and an apparatus for producing heat set polyester films in an advantageous manner from a biaxially oriented tubular polyethylene terephthalate film.
It is necessary that packaging films have good dimensional stability. This dimensional stability is given to flat webs of biaxially oriented polyethylene terephthalate films by a heat setting process. Usually heat setting is carried out in a tenter stretching process wherein the film web is stretched and then heat set by hot air while being supported at its edges by means of clips of the tenter device used in the process.
On the other hand, in a tubular stretching process, heat setting by enclosing the air in the interior of a tubular film and expanding the resulting bubble by air pressure is known, as are heat setting by insertion of a mandrel in the interior of the film tube and sliding the tube over the mandrel, and heat setting a tubular film in a folded state in an oven after stretching.
In the process for heat setting a tubular film while expanding with internal air pressure, it is difficult to maintain the film bubble in a definite shape because the tubular film to be heat set is unsupported and a considerable film shrinkage stress occurs during heating; consequently it is difficult to carry out continuously stable heat setting of the films. There is the further problem in that physical properties which are uniform just after stretching are rather injured by the heat setting processing because of sway of the tubular film during heat setting.
Accordingly, heat setting of biaxially oriented tubular films has not been carried out advantageously industrially because of having the above described defects, even though the prior art of tubular heat setting is existent.
In Japanese Patent Publication No. 12,038/1968 there is disclosed a process which comprises heat setting a tubular film by heating means which are divided into multistage rooms along the running direction of the tubular film, wherein a suitable pressure is applied to each room, while an exterior pressure is applied to the tubular film so that the diameter of the tubular film after the heat processing means is smaller than a diameter of the stretched tubular film just prior to heat setting. However, this process is difficult to put in practical use because control of the pressure is complicated.